Synopsis

A couple of criminals named Flex and Slim are robbing a museum. They are wearing sweats similar to what people at a gym would wear. As they walk around they miss the fact that they are being watched by Darkwing and Launchpad. As they are attempting to steal a helmet off of a suit of armor, Darkwing makes his presence known. Flex throws the helmet at Darkwing and Launchpad catches it. The momentum of the helmet pushes Launchpad out the window and Darkwing goes to grab him. Just as Darkwing grabs launchpad, Slim throws a barbell and hits Darkwing knocking them out the window. Darkwing uses a flagpole on the building to stop their fall and it slings them back through the window. Unfortunately they got away but the barbell left an imprint on Darkwing's behind and he recognizes the logo as a local gym. Darkwing decides to go investigate the gym as Drake Mallard.

As he is packing the car, Herb and Binkie Muddlefoot show up and tell him that they are joining the gym too. Binkie has a three-for-one coupon.

Meanwhile Flex and Slim report to their boss Jock Newbody. It turns out he is 122 years old. The items that they were stealing were to make a fountain of youth so he could become young again and extend his life. It turns out that the feather on the helmet that Flex threw at Darkwing was the last ingredient that he needs. A feather from a true and pure hero. Slim explains that Darkwing foiled their attempt and that he may follow them back to the gym. The boss is happy to learn because he can use a feather from Darkwing to complete the formula. Slim and Flex know that Darkwing will come to the club in a disguise and they have to try and figure out which applicant he really is.

Drake, Herb, and Binkie drive up and Binkie tells them that they are here to join the club. Slim starts them exercising immediately and Herb is ready to go already. Flex and Slim try and figure out which one is Darkwing when Darkwing shows up and finds out that they have a boss. He easily beats them in a fight and goes to try and find their boss. Flex and Slim think that Herb is Darkwing when they see him running outside (he is actually trying to get out of the club). They tackle him and get one of his feathers. They head off to their boss and Darkwing is about to follow them when Herb accidentally lands in the guard dog pen. Darkwing jumps in to save him and as they try to get out, they run into an electric fence that melts their wrist weights together.
Flex and Slim bring the feather to their boss and he adds it to the formula and of course, with it being the feather of Herb, it does not work. The boss yells at them to find Darkwing.

Darkwing and Herb are now stuck to each other and Darkwing cannot get them free. They are captured by Slim and Flex and are taken to the boss. Slim plucks a feather from Darkwing and the boss adds it to the formula. This time it does work and makes him young once again. Darkwing tries to kick him but only hurts his foot in the process. The boss tells them that he will have to kill them since they now know his secret including Flex and Slim. In order for them to escape the trap, Herb must do a sit up so Darkwing can reach the ropes to untie himself. Herb tries and cannot do it. Darkwing tells him to do it for Binkie. Herb is able to do the sit up and they get out of the trap.

They find the boss putting the formula in a drum and try to stop him. They all wind up landing in the formula making them all babies. Darkwing and the boss race to get the antidote to change back when Slim and Flex show up to stop them. They capture Drakwing and the boss and Herb slips through. They start to take Darkwing and the boss to do away with them when Herb rides the table with explosives into the room and knocks Slim and Flex into the fountain and it explodes.

All of the formula is gone and Darkwing gets the antidote and uses it on himself and Herb. The baby bad guys are locked up and Darkwing thanks Herb for saving his life. Herb goes to tell Binkie what happened and runs into her and finds that she looks really good now. Drake walks in and Herb tells him all about his adventure.

Trivia

When Drake drives up to the gym for the first time, he is wearing his Darkwing mask

When Darkwing Duck, Herb and the villain fall into the fountain, the villain says "You big meanie! Look what you did!" Darkwing should have nothing on but his hat, mask, diaper and cape but when he says that he is wearing his full outfit.